Current work in this laboratory suggests that vitamin D may influence the metabolism of parathyroid hormone (PTH). Fragmentary evidence indicates that the administration of 1,OH-D3 to patients with renal failure induces a marked change in the immunological characteristics of circulating PTH. Using the isolated perfused rat liver, we will systematically explore this problem. This laboratory is well equipped for this study since the perfused liver has been used by us in the past for the study of PTH metabolism: A large body of background information concerning the kinetics of fragment generation has been accumulated. In addition, there now exists a solid body of information concerning the immunologic and biologic characteristics of these fragments. The biologic significance of PTH fragments remains obscure. In particular, does this represent an activation process or a degregation process? This problem will be carefully investigated using a variety of test systems and a variety of naturally produced and synthetic fragments.